Same UY and resort will show as a combined total on the main online page. When you book you can choose which one to use first. However if you want to use some from one and some from another, you have to call.
For example, you desired ressie is 100 points. You use the 75 remaining in your current UY on the first contract and 25 from the current UY from the 2nd. That can be done online as the 75 depleted that UY's points from that contract and then the system knows to look elsewhere. But if you wanted to use 50 from one and 50 from the other and that left points still available in both UY's, per contract, then you'd have to call.
For the same resort and UY I can't see where it would really matter but same UY and different resorts it would, or could, at 11 months.
Basically the online system doesn't let you choose xx number of points, it just goes for current UY whatever is available and uses all of that first, it will then go to borrowing or current UY other contacts. If you had current UY points available in both it should let you pick which one to use first. Anything more complicated you have to call.